[Tinyos-help] Why is sendDone() called twice?

[Tarun Bansal]

I was examining the behavior of sendDone() after sending a packet over
radio (in TOSSIM environment). I noticed that it is being called twice
for every packet sent.

If I send the pkt to an actual address, both sendDone()'s  have their
ack flag up.
However if the pkt is sent to a fictitious mote, the flag is first up
and second time down.

Can you please explain this behaviour and how do I interpret these readings?
Also should I expect the same behaviour on real Mica2 motes?

Tarun